This invention relates to a dot signal conversion method which converts image signals of three primary colors into four color dot signals in a color image input/output system.
In a color image input/output system incorporating a lay-out scanner, a direct scanner, etc., a color manuscript is generally read out for color separation by a reading device, input density signals of the respective colors from the reading device are processed appropriately for colors, and then are converted into dot signals to be fed to an image output device.
FIG. 1 is a block diagram to show the processing flow from input image signals 1 (in three colors of Y (yellow), M (magenta) and C (cyan)) to dot signals 7. The minimum values, Min (Y, M, C) of the colors Y, M and C are obtained from the input image signals 1 by a minimum value selecting section 2 to be used in calculation by a dot coefficient calculating section 3 in accordance with equation (1) below. EQU U.sub.R =(Min (Y, M, C)-TH.sub.R)+ (1)
wherein
U.sub.R : dot coefficient PA0 Min (Y, M, C): the minimum values of density for Y, M and C. PA0 TH.sub.R : starting point of calculation of dot intensity. PA0 ()+: positive numbers in the result of calculation. PA0 K.sub.Y, K.sub.M, K.sub.C : dot intensity (full black (ICR) when 0.ltoreq.K.ltoreq.1, K=(1).
The calculation result is fed to a dot calculating section 4 for respective colors together with the input image signals 1, and they are calculated according to equation (2) below. ##EQU1## wherein Y', M', C': dot signals
The minimum density signals Min (Y, M, C) of respective colors obtained from the minimum value selecting section 2 are sent to a black print dot calculating section 5 for the calculation with the following equation (3). EQU B.sub.K '=f(Min(Y, M, C)) (3)
wherein f: black print gradation table. Dot signals 7 are finally obtained in the four colors of Y', M', C' and B.sub.K (black).
When an operator runs the system having such a processing flow, he sets predetermined parameters TH.sub.R, K.sub.Y, K.sub.M, K.sub.C and f, to obtain predetermined dot signals, or in other words, not to disturb the gray balance of the images. However, the number of parameters which should be set by the operator is large in this prior of method, and changes and adjustments among these parameters involve complicated operations. For instance, if one whishes to increase the percentage of black print (B.sub.K) beyond the ordinary rate, one must change not only the black print gradation table but also K.sub.Y, K.sub.M and K.sub.C, and yet it is extremely difficult to change all of these parameters without disturbing gray balance unless the operator has considerable skill and experience.